1. Field of Invention
The invention relates to a method for operating a resonance measuring system, in particular, a Coriolis mass flowmeter, wherein the resonance measuring system includes at least one oscillation element interactive with a process, at least one oscillation driver and at least one oscillation sensor, wherein the oscillation element is excited to oscillation in at least one eigenform with known excitation signals Fi(t) by the oscillation driver and the oscillation element is excited to oscillation with unknown excitation signals W(t) by the process and the oscillations of the oscillation element are detected by the oscillation sensor and are formed as at least one response signal yi(t) of the respective eigenform. In addition, the invention relates to a resonance measuring system that is operated using such a method.
2. Description of Related Art
Resonance measuring systems of the type mentioned above have been known for years, not only in the form of Coriolis mass flow meters, but also in density-measuring devices or level monitors using the tuning fork principle, in quartz scales and band viscometers, among other things. These resonance measuring systems are connected with a process, wherein the process and the resonance measuring system are interactive. In the exemplary Coriolis mass flowmeter, the process consists of a medium flowing through the oscillation element—Coriolis measuring tube, wherein the actual measurement of significant interest is the mass flow through the oscillation element. The influence of the process through the oscillation element—or, more generally, through the resonance measuring system—is usually negligible. The eigenfrequencies of the resonance measuring system, or, respectively the oscillation element are, moreover, of particular importance because preferred working points of the resonance measuring system are placed at eigenfrequencies of the measuring tube in order to be able to imprint the necessary oscillations for induction of Coriolis forces at a minimal energy input.
Here, and in the following, it is of particular interest that, in a Coriolis mass flowmeter, further additional information, namely information about the density of the fluid of the flowing medium, is encoded in the eigenfrequencies of the oscillation element.
In the following, resonance measuring systems are covered using the example of Coriolis mass flowmeters, but it should be understood as not being limited to Coriolis mass flowmeters. Such systems are generally termed resonance measuring systems, in which information about the determining process variables (indicators) is encoded in the eigenfrequencies and/or such systems in which the working points are placed at the eigenfrequencies of the measuring system. The further developments described in the following can be used on all systems subject to this definition.
The oscillation element of the resonance measuring system is generally excited to oscillation in an eigenform using a known excitation signal Fi(t)—or using multiple known excitation signals Fi(t). The number of excitation signals required is dependent on the eigenform to be excited and from the number and placement of oscillation drivers along the oscillation element. For simplicity, generally one excitation signal Fi(t) will be mentioned, but there could always be multiple excitation signals. On this note, the oscillations of the oscillation element are detected with one or with multiple oscillation sensors, which leads to one response signal or to multiple response signals; in the following, for simplicity, usually just one response signal will be mentioned, whereby—as mentioned above—there could also always be multiple response signals. In both cases, this is not to be understood as limiting.
The known excitation signal Fi(t) can be called known insofar as that it is created, for example, by a conventionally used control. This excitation signal is often a harmonic signal, i.e., e.g., a sinusoidal force stimulus by means of the oscillation driver. In addition to the known, imprinted excitation signals Fi(t), the process influences the oscillation element. This influence occurs, on the one hand, via changes in the parameters of the oscillation elements (parametric excitation), and on the other hand, via the excitation of the oscillations elements using unknown excitation signals W(t).
Known mass flowmeters, which work according to the Coriolis principle, are characterized by a high accuracy and reliability in a one-phase flow mode—i.e., flow of a physically homogenous medium. However, this does not hold true for multi-phase flows. A multi-phase flow is, in general, a flow that is composed of at least two phases having different physical characteristics. Here, the phases could be either of the same or of different materials. Homogenous and spatially limited areas are signified as phases. Some examples are liquid-solid flow, gas-liquid flow, gas-solid flow, water-steam flow or water-air flow. Flows that, for example, occur in filling, emptying, process-starting and switching of valves are to be understood, here, as multi-phase flows.
In using multi-phase flows, considerable measuring errors occur. The essential cause, there, is the occurrence of an asymmetrical filling of the measuring tube during flow, which leads to a rapid fluctuation of the resonance frequency, as well as the occurrence and cessation of secondary flows in the measuring tube, which cause a quick attenuation at occurrence and a swift disattenuation at cessation. Generally, secondary flows are caused by differing densities of the flow phases.
A loss of the working point occurs in current, relatively slow—based on the speed of the instationariness of the flow—operating and controlling modes due to quick attenuation and disattenuation of the measuring tube as a result of an instationary flow and a simultaneously rapid change of the resonance frequency. Then, the maximum available output is often not enough to maintain the oscillations of the measuring tube. The result is that no Coriolis forces can be induced, whereby the measurability of the mass flow is lost. The drive power cannot be arbitrarily increased and is actually limited in existing resonance measuring systems; on the one hand, for the purpose of explosion protection, so that not too much energy is accumulated in the system, and on the other hand, so that no unfavorably high oscillation amplitudes occur in the case of interruption of the multi-phase flow.
Above all, the positioning of the working point in resonance measuring systems having poorly damped eigenforms is problematic as, e.g., in many Coriolis mass flowmeters, in which a resonance increase of the measuring tube oscillation can exist that is higher than a magnitude of three. This clarifies the high requirements that are to be placed on the adaptation of the excitation signal. The loss of the working point is associated with a restart—partly time-consuming and energy demanding—of a resonance measuring system.
Furthermore, the loss of the working point results in that the process variables of interest related to the eigenfrequency are no longer known, so that, here, another abrupt loss of information exists.